This invention relates to methods for synthesis of organic compounds, and in particular to compounds useful as protecting and linking groups for use in the synthesis of peptides, oligosaccharides, glycopeptides and glycolipids. The invention provides protecting and linking groups which are useful in both solid phase and solution synthesis, and are particularly applicable to combinatorial synthesis.
The problem of functional group incompatibility in the synthesis or complex organic structures demands the use of a functional group protection strategy. Complex synthetic intermediates and products usually contain a multiplicity of reactive groups, most of which must first be blocked, and subsequently liberated at an appropriate point in the synthesis . The problem is especially acute in the design and construction of polyfunctional molecules such as oligosaccharides, peptides, glycopeptides and glycolipids.
In oligosaccharide synthesis, a variety of protective groups are required. It is necessary to place groups regioselectively at specific locations; on primary alcohols, on cis-diols, on trans-diols, on 1,2-diols, on 1,3-diols, or on particular secondary alcohols. In addition, aminosugars are important constituents of oligosaccharides, and their amino-protection should be compatible with the hydroxy group protection strategy. The properties of the protective group adjacent to the anomeric centre are also important. Whether this group is participating or non-participating plays a significant role in control of glycoside stereochemistry. Because most reactions at the glycosidic centre proceed via electron deficient intermediates, electron-releasing substituents on the C-2 substituent accelerate the reaction at the glycosidic centre. Electron-withdrawing substituents, normally esters or amides, slow the reaction. In solid phase oligosaccharide synthesis, the stability and sensitivity of the linker between the first sugar unit and the resin becomes a crucial part of the protection plan. The presence of other functional groups, such as alkenes or esters, or features such as a furanose ring in the target oligosaccharide, may dictate that the protecting groups used for the synthesis are not sensitive to acid, base, reductive, or other commonly used cleavage techniques. The choice of protecting groups is therefore one of the decisive factors in the successful realization of solid phase oligosaccharide synthesis.
In solid phase peptide and glycopeptide synthesis the demand of a new orthogonal protective set is significant. The established orthogonal deprotection sets are based upon the well-known Fmoc and Boc protection of amino acids. The construction of complex peptides or glycopeptides often requires a third orthogonal protecting group for side-chain amino functionalities, whose removal will not affect the protecting groups in the other orthogonal sets, or vice versa.
Many protecting groups have been developed for amino group protection, and fall into seven broad classes.
1. N-Acyl Derivatives
a) Phthalimides are especially useful in the protection of amino functions in aminoglycoside synthesis (Nicolaou et al, 1992), because they are stable during the glycosylation, and because they help to control the stereochemistry by neighbouring group participation. Unfortunately, the deprotection needs vigorous conditions, which often results in partial product decomposition.
b) Trifluoroacetamides (Weygand and Czendes, 1952) Simple amide derivatives are usually worthless as protecting groups because the conditions required to remove them are too harsh. However, the trifluoroacetamide group is exceptionally labile to base hydrolysis, and is therefore useful in the protection of amines.
c) Carbamates are used as protective groups for amino acids to minimize racemization in peptide synthesis. Racemization occurs during the base-catalysed coupling reaction of an N-protected, carboxyl-activated amino acid, and takes place via the intermediate oxazolone that forms readily from an N-acyl protected amino acid. Many carbamates, for example Boc (McKay and Albertson, 1957), Cbz (Bergman and Zervas, 1932), Alloc (Kunz and Unverzagt, 1984), Teoc (Carpino et al, 1978), and Troc (Windholz and Johnston, 1967), have been used as protective groups for amino protection.
2. N-Sulfonyl Derivatives
Sulfonamide derivatives are frequently used in nitrogen heterocycles (Gribble et al, 1992), and arylsulfonyl (Fischer and Livschitz, 1915) groups are effective protective groups for a wide range of primary and secondary amines, but their deprotection requires drastic conditions. xcex2-(Trimethylsilyl)ethanesulfonyl (Weinreb et al, 1986) derivatives are as stable as arylsulfonyl groups, but the cleavage step requires only gentle warming with TBAF or CsF.
3. N-Sulfenyl Derivatives
Sulfenamides are much more labile than sulfonamides, being sensitive to acids as well as to attack by nucleophiles. Their deprotection requires exceptionally mild conditions. Several sulfenyl groups are used for the protection of the amino function including tritylsulfenyl (Brandchaud, 1983), o-nitrophenylsulfenyl (Goerdeler and Holst, 1959), and pentachlorphenylsulfenyl (Kessler and Iselin, 1966).
4. N-Alkyl Derivatives
Benzylamines give useful protection in reactions in which metal hydrides are used and the carbamates are not stable. Benzylamines are less susceptible to catalytic hydrogenolysis than benzyl ethers or benzyl esters, and thus selective deprotection can often be achieved (Goldstein et al, 1992). The trityl group (Sieber and Riniker, 1991) is used to protect amino acids, although its steric bulk and high acid lability is detrimental to peptide coupling. The 9-phenylfluorenyl (PhFl; Koskinen and Rapoport, 1989) group is used for the protection of primary and secondary amines. Its hydrophobicity, steric bulk and ease of introduction are similar to the trityl group, but the PhFl group is about 6000 times more stable to acid than the trityl group.
5. N-Silyl Derivatives
The high acid and moisture sensitivity of silylamines has been a major obstacle to their use in amino group protection. Butyldiphenylsilylamines (Overman and Okazaki, 1986) have remarkable stability towards strong basic conditions, but they are still very acid labile.
6. Imine Derivatives
The double bond of the imine function allows for the simultaneous protection of both Nxe2x80x94H bonds of a primary amine. Imines are generally stable towards strongly basic conditions, but they are labile to aqueous acid. N-Silyl imines (Colvin et al, 1988), N-bis(methylthio)methyleneamines (Hoppe and Beckmann, 1979) and N-diphenylmethyleneamines (Polt et al, 1979) are valuable for the protection of amino groups in the synthesis of xcex1-amino acids.
7. Enamine Derivatives
N-(5,5-Dimethyl-3-oxo-1-cyclohexenyl)amine (Halpern and James, 1964) is used to protect amino acids, giving vinylogous amide derivatives. These compounds can be cleaved by treatment with either aqueous bromine or nitrous acid. The stability of the vinylogous amide-protected primary amines mainly depends on the structure of 1,3-dione and the functional group attached to the enamine double bond. The open chain N-(4-oxopent-2-enyl)-protected amines are labile towards aqueous and mildly acidic conditions. This acid sensitivity limits their use as synthetic reagents (Kellam, 1996). The cyclic 1,3-diketone, 5,5-dimethylcyclohexane-1,3-dione (dimedone) reacts with dimethylformamide dimethylacetal affording 5,5-dimethyl-2-(dimethylaminomethylene)cyclohexane-1,3-dione. Bycroft et al (1993) used this reagent to synthesise Dmc-protected xcex1-amino acids, and found remarkable stability towards acidic conditions. The deprotection of these compounds could be rapidly achieved by a dilute hydrazine solution at room temperature. The introduction of a methyl group to the enamine double bond provided the N-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl Dde-protective group, improving the stability towards secondary amines (Bycroft et al, 1993). The N-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-3-methylbutyl-protected amino acids (Chan et al, 1995), carrying a bulkier group at the enamine double bond, had excellent base stability. N-1-(4-Nitro-1,3-dioxoindan-2-ylidene)-ethyl (Nde; Kellam, 1996; Mosher and Meier, 1970) protection of amino acids gave similar vinylogous systems, and deprotection of these could be achieved in very mild conditions.
For many years chemists have attempted to transpose the solid-phase methodology which is routinely used for peptide synthesis to oligosaccharide synthesis, with varying degrees of success. The first attempt was approximately 25 years ago (Frechet and Schuerch, 1971; Frechez and Schuerch, 1972; Guthrie et al, 1971; Guthrie et al, 1973). However, the ozone-mediated deprotection product was an aldehyde-substituted glycoside. Danishefsky and coworkers described the solid phase synthesis of the Lewis b Antigen (Randolph et al, 1995) and N-linked glycopeptides (Roberge et al, 1995) by initial attachment of the primary sugar unit of the oligosaccharide to a 1% divinylbenzene-styrene co-polymer support via a silyl ether linkage. The resin-bound sugar moiety was in this instance a glycal, with on-resin activation achieved via epoxidation of the double bond, and the resulting glycal residue acting as a sugar donor through nucleophile ring-opening of the epoxide. Since there are no colorimetric methods available to the sugar chemist to monitor on-resin glycosylations, the only means of assessing the progress of the reaction is by lysis of the oligosaccharide-resin bond and subsequent analysis of the cleavage product, usually by thin layer chromatography. The tetra-n-butylammonium fluoride-mediated deprotection conditions required to cleave Danishefsky""s silyl ether linker are both hazardous and slow. This, coupled with the requirement for on-resin activation of the tethered glycals, makes the overall strategy and methodology far from ideal.
In an alternative approach, Douglas and coworkers described the synthesis of D-mannopentose using a polyethyleneglycol w-monomethylether co-polymer and a succinoyl or an xcex1,xcex1xe2x80x2-dioxyxylyl diether linker (Douglas et al, 1995). The reactions were carried out in solution phase, with removal of unused reactants being achieved by precipitation of the oligosaccharide-polymer complex and subsequent washing. In the latter example, cleavage of the oligosaccharide-polymer bond was achieved through catalytic hydrogenation, which required exposure of the conjugate to 1 atm of H2 for 48 h to achieve respectable yields. This again is far too slow to allow effective monitoring of individual glycosylation reactions. Yan et al reported sulphoxide-mediated glycosylation on a Merrifield resin, using a thiophenol linker for the attachment of the primary sugar residue (Yan et al, 1994). This method resulted in the construction of (1-6)-linked oligosaccharides, and was suitable for synthesis of both xcex1- and xcex2-glycosidic linkages. However, the thioglycosidic linkage to the resin dictates that similar sugar donors cannot be employed in this strategy.
Recently Rademann and Schmidt reported the use of trichloroacetimidate sugar donors to a resin bound sugar tethered via an alkyl thiol (Rademann and Schmidt, 1996); once again, however, this method precludes the use of the far superior thioglycoside sugar donors. Meanwhile, Adinolfi et al described the synthesis of disaccharides using a polyethyleneglycol-polystyrene resin, with connection of the first sugar to the polymeric support through a succinate spacer (Adinolfi et al, 1996). However, the acid lability displayed by this linker means that the primary sugar cannot be linked to the resin via the glycosidic position.
These examples illustrate that the critical element in solid phase synthesis is the nature of the linker between the solid support and the initial synthon. The linker must display excellent stability to the conditions of coupling and deprotection, yet in the case of solid phase oligosaccharide synthesis, it should also be rapidly and efficiently cleaved to allow monitoring of the progress of individual coupling reactions. The cleavage should ideally be achieved by the use of a relatively innocuous chemical reagent. There remains a need in the art for simple, efficient and economical methods for solid-phase synthesis of oligosaccharides.
In our allowed U.S. Pat. No. 6,523,337, which is the U.S. national stage application of International Patent Application No. PCT/AU97/00544 (priority date Aug. 28, 1996), we have shown several ways of immobilizing 2-acyl-5,5-dimethyl-1,3-cyclohexanedione and of utilizing the immobilized compound in solid phase oligosaccharide synthesis. In our U.S. Pat. No. 6,462,183, which is the U.S. national stage application of International Patent Application No. PCT/AU98/00131 (priority date Feb. 28, 1997), we have shown that vinylogous amide protection of amino sugars could be achieved in simple reactions using Dde-OH and Nde-OH reagents. The Dde- and Nde-protected monosaccharides survived most of the hydroxyl protective group manipulations and the reactions which occurred at the glycosidic center, affording a wide variety of sugar donors. These vinylogous amide-protected aminosugar donors were not neighbouring group active carbohydrates, giving anomeric mixtures of glycosides during the glycosylations. We have demonstrated the stability and the ease of deprotection of the Dde- and Nde-protected aminosugars in carbohydrate-based methodology.
Unfortunately even these protective strategies still present some difficulties.
The Dde-protected aminosugars are not stable in the presence of sodium cyanoborohydride and metal hydrides. These reagents are often used in benzylidene ring opening reactions and during benzyl protection of hydroxyl groups. This hydride sensitivity of the Dde group limits its application in carbohydrate chemistry. The preparation of 2-acyl-dimedones is very often difficult. One of the major side reactions is O-acylation, which lowers the overall yields and causes difficult chromatographic purification problems.
Nde-protection of primary amines always gives a mixture of E/Z isomers which may not be separable, causing difficult characterisation problems. The formation of 2-acetyl-4-nitroindan-1,3-dione involves the reaction between 4-nitrophthalic anhydride and 2,4-pentanedione via a condensation and two rearrangements. This synthetic strategy does not give an opportunity to prepare Nde-OH analogues.
We have now synthesized a family of novel compounds useful as protecting and linking groups for organic synthesis.
In its most general aspect, the invention provides a cyclic compound of general formula I 
wherein the ring is a cycloalkyl, substituted cycloalkyl, cycloheteroalkyl, substituted cycloheteroalkyl, saturated bicyclo[p, q, r], substituted saturated bicyclo[p, q, r], saturated heterobicyclo[p, q, r], substituted saturated heterobicyclo[p, q, r], unsaturated bicyclo[p, q, r], substituted unsaturated bicyclo[p, q, r], unsaturated heterobicyclo[p, q, r], substituted unsaturated heterobicyclo[p, q, r], saturated tricyclo[p, q, r, s], substituted saturated tricyclo[p, q, r, s,], unsaturated tricycloalkyl[p, q, r, s], unsaturated substituted tricycloalkyl[p, q, r, s], saturated heterotricyclo[p, q, r, s,], substituted saturated heterotricyclo[p, q, r, s,], unsaturated heterotricyclo[p, q, r, s,] or substituted unsaturated heterotricyclo[p, q, r, s,] ring system; where p, q, r and s may be the same or different, and each of p, q, r and s is an integer of from 0 to 5;
X is oxygen, sulphur, imino or substituted imino;
R1 is hydrogen; an alkyl, alkenyl, alkynyl, heteroalkyl, aryl, heteroaryl, cycloheteroaryl, cycloalkyl, heterocycloalkyl, alkanal, or thioalkanal group, each of which may be substituted or unsubstituted; NH2, guanidino, CN, substituted amino, quaternary ammonium, Oxe2x88x92, formyl, imino or substituted imino, COOH, or a carboxylic acid derivative;
R2 is an alkylamino, dialkylamino, arylamino, or diarylamino group, each of which may be substituted or unsubstituted; O-substituted hydroxylamino, substituted or unsubstituted hydrazino, substituted or unsubstituted hydrazido, substituted or unsubstituted thiohydrazido, semicarbazido, thiosemicarbazido, OH, Oxe2x88x92M,NH2, NHOH, SH, Sxe2x88x92M+, halogen; O-alkyl, O-acyl, O-aryl, alkylthio, S-aryl, acylthio, alkylsulfonyl or arylsulfonyl, each of which may be substituted or unsubstituted; and M is a metal ion, or an organic or inorganic cation such as a quaternary amine group, a trityl group or an ammonium group,
with the provisos that the compound is not one disclosed in our U.S. Pat. No. 6,573,337 which is the U.S. national stage application of International Patent Application No. PCT/AU97/00544.
A wide variety of suitable cations is known in the art. The metal ion can be mono- or multivalent, and may form a complex salt.
Preferably the ring is 4- to 8-membered cycloalkyl, substituted cycloalkyl, cycloheteroalkyl or substituted cycloheteroalkyl.
Alternatively in other preferred forms, the ring is a 5- to 8-membered ring of the lactone or lactam type, or a 6- to 8-membered ring of the carbamido or substituted carbamido type, as follows: 
in which each R is independently H, substituted or unsubstituted alkyl, aryl, alkenyl, alkynyl or acyl, or may be a 6- to 8-membered ring of the carbonate type, as follows: 
It will be clearly understood that in the general formulae of this specification, each of the substituent groups R, R1, R2 and R3 may itself be substituted, ie. one or more hydrogen atoms may be replaced by a substituent group.
For the purposes of this specification the term xe2x80x9csubstitutedxe2x80x9d in the definitions of R, R1 and R2, and in definitions of other substituents within this specification, means that the substituent is itself substituted with a group which modifies the general chemical characteristics of the chain. Preferred substituents include but are not limited to halogen, nitro, amino, azido, oxo, hydroxyl, thiol, carboxy, carboxy ester, carboxyamide, alkylamino, alkyldithio, alkylthio, alkoxy, acylamido, acyloxy, or acylthio, each of 1 to 3 carbon atoms. Such substituents can be used to modify characteristics of the molecule as a whole, such as stability, solubility, and ability to form crystals. The person skilled in the art will be aware of other suitable substituents of similar size and charge characteristics which could be used as alternatives in a given situation.
In one group of preferred embodiments, the compound is of general formula II 
in which each R is independently H or a substituted or unsubstituted alkyl, aryl, cycloalkyl, heteroalkyl, heteroaryl or heterocycloalkyl; and
R1 and R2 are as defined in general formula I.
Preferably each R has 1 to 6, more preferably 1 to 4 carbon atoms.
In another group of preferred embodiments, the compound is of general formula III 
in which
R1 and R2 are as defined in general formula I.
The compounds of the invention are useful in a wide variety of areas of organic chemistry. The compounds are especially useful in the solution and/or solid phase synthesis of oligosaccharides and peptides. Uses of the compounds of the invention thus include but are not limited to the following:
1. Linker groups for solid-phase oligosaccharide synthesis;
2. N-protecting groups for protection of amino sugars in oligosaccharide synthesis;
3. Linker groups for solid phase organic synthesis;
4. N-protecting groups for organic synthesis;
5. N-side chain and/or Nxcex1 protecting groups for solid or solution phase peptide synthesis;
6. Amino protecting groups for sugars, peptides and organic compounds, affording an additional free enamine;
7. Certain compounds of the invention are chiral; these are useful in resolution or enantiomers and in stereospecific synthesis.
8. Linker groups for coupling of a starter group to a resin or solid phase synthesis of oligosaccharides, peptides and other organic compounds.
Thus in a second aspect, the invention provides an N-protecting group for oligosaccharides, amino acids, peptides or organic compounds.
An example of the application of this group for the protection of amino groups during oligosaccharide synthesis is shown in general formula IV 
wherein the ring, X and R1 are as defined in general formula I, and
R3 is a protected, unprotected or substituted sugar amino-, a glycosylamino-, or a glycosylamino group of an oligosaccharide; or a mono- or oligosaccharide coupled through a substituted or unsubstituted alkylamino-, arylamino-, cycloalkylamino, heteroalkylamino, heteroarylamino or heterocycloalkylamino group.
In one group of preferred embodiments, the compound is of general formula V 
in which R and R1 are as defined in general formula II, and R3 is as defined in general formula IV.
Preferably R3 is a protected, unprotected or substituted sugar amino-, a glycosylamino-, or a glycosylamino group of an oligosaccharide.
Alternatively, R3 is an oligosaccharide-Oxe2x80x94CH2xe2x80x94(C6H4)xe2x80x94NHxe2x80x94, monosaccharide-Oxe2x80x94CH2xe2x80x94(C6H4)xe2x80x94NHxe2x80x94, oligosaccharide-CO2CH2xe2x80x94(C6H4)NHxe2x80x94, or monosaccharide-CO2CH2xe2x80x94(C6H4)xe2x80x94NH group.
In a third aspect the invention provides a support of general formula VI for solid-phase synthesis of oligosaccharides, peptides or organic compounds, comprising a resin and a linker covalently attached to the resin: 
wherein the ring, X and R2 are as defined in general formula I, and
R1 is a substituted or unsubstituted alkyl, cycloalkyl, heteroalkyl, heteroaryl, heterocycloalkyl or carboxylamido spacer group which is directly coupled to the resin support, or which may optionally be coupled to the resin support via a suitable covalent linkage, which is stable to conditions of oligosaccharide synthesis and cleavage.
The covalent linkage may suitably be provided by a xe2x80x94CONHxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94COSxe2x80x94, xe2x80x94CHxe2x95x90Nxe2x80x94, xe2x80x94NHCONHxe2x80x94, xe2x80x94NHCSNH, xe2x80x94NHNHxe2x80x94 grouping, eg. Spacer-CONH-resin, Spacer-O-resin, Spacer-S-resin, Spacer-S-S-resin, Spacer-CO2-resin, Spacer-CHxe2x95x90N-resin, Spacer-NHCONH-resin, Spacer-NHCSNH-resin, Spacer-NHNH-resin. Other possible covalent linking groups will be known to those skilled in the art.
In a particularly preferred embodiment, the linker is a barbituric acid of general formula VII 
in which R and R2 are as defined in general formula I, and R1 is as defined in general formula VI,
in which a compound of general formula II is directly coupled to the resin support, or may optionally be coupled to the resin support via a suitable covalent linkage which is stable to conditions of oligosaccharide synthesis and cleavage.
The covalent linkage may suitably be provided by a xe2x80x94CONHxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94COSxe2x80x94, xe2x80x94CHxe2x95x90Nxe2x80x94, xe2x80x94NHCONHxe2x80x94, xe2x80x94NHCSNH, or xe2x80x94NHNHxe2x80x94 grouping, eg. Spacer-CONH-resin, Spacer-O-resin, Spacer-S-resin, Spacer-S-S-resin, Spacer-CO2-resin, Spacer-CHxe2x95x90N-resin, Spacer-NHCONH-resin, Spacer-NHCSNH-resin, Spacer-NHNH-resin. Other possible covalent linking groups will be known to those skilled in the art.
The resin may be any resin which swells in water and/or in an organic solvent, and which comprises one of the following substituents: halogen, hydroxy, carboxyl, SH, NH2, formyl, SO2NH2, or NHNH2, for example methylbenzhydrylamine (MBHA) resin, amino or carboxy tentagel resins, or 4-sulphamylbenzyl AM resin. Other suitable resins will be known to those skilled in the art. Alternatively, supports such as controlled-pore glass or soluble polymer supports may be used. These are well known in the art.
The invention also provides a method of solid-phase synthesis of oligosaccharides, comprising the step of sequentially linking mono- or oligosaccharide groups to a support as described above.
The linker may be synthesised directly on the resin in a stepwise manner prior to the coupling of the initial sugar group, or the linker-initial sugar conjugate may be synthesised in solution phase and subsequently coupled to the solid support, with subsequent sugars being sequentially attached. Preferably the second and all subsequent sugar groups are coupled to the oligosaccharide chain-resin conjugate after the last sugar in the oligosaccharide chain is partially deprotected.
The first sugars attached to the resin-linker unit may be unprotected, partially protected or fully protected glycosides, aminoglycosides, or ether- or amino-linked sugars.
Preferably the first sugar coupled to the resin is an aminosugar, an aminoglycoside or an amino-oligosaccharide, or a glycosyl amines of an oligosaccharide.
In one particularly preferred embodiment the support comprises a resin, a linker and a saccharide selected from the group consisting of monosaccharide, oligosaccharides, or aminosaccharides and aminooligosaccharides.
The building block mono- or oligosaccharide-donors may be any activated sugar, including but not limited to orthoesters, thio-orthoesters, cyanoalkylidene derivatives, 1-O-acyl sugars, amino sugars, acetimidates, trichloroacetimidates, thioglycosides, aminoglycosides, aminoligosaccharides, glycosylamines of oligosaccharides, glycosyl thiocyanates, pentenyl glycosides, pentenoylglycosides, isopropenyl glycosides, glycals, tetramethylphosphoro diamidates, sugar diazirines, selenoglycosides, phosphorodithioates, glycosyldialkylphosphites, glycosylsulphoxides and glycosylfluorides.
Preferably partial sugar deprotection is achieved by using acyl-type, trityl, methoxytrityl, methoxybenzyl, various silyl and/or photolabile protecting groups in addition to permanent ether-type protecting groups. This permits the synthesis of branched oligosaccharides by using two orthogonal hydroxy-protecting groups on a single sugar donor.
The synthesised oligosaccharide can be cleaved from the resin using ammonia, hydrazine or a primary amine, such as butylamine or cyclohexylamine. For the preparation of aminoglycosides, ammonia or a suitable primary amine in an organic solvent is preferably employed. For the preparation of hydrazides, hydrazine in water or an organic solvent is preferably employed. For the preparation of oligosaccharides, ammonia in water or organic solvent is preferably employed, followed by acidification. When the linker contains a 4-aminobenzyl moiety, after cleavage as described above the first sugar is released still protected by the aminobenzyl group; this can be removed by hydrogenation if desired.
In a preferred embodiment, the invention provides a reagent for solution phase synthesis of sugar-containing compounds, comprising a barbituric acid derivative compound of general formula II as defined above.
The compounds of the invention are suitable for use as protective groups in methods of solid phase oligosaccharide synthesis, in which sugar units are linked to a resin. Any suitable linker compound may be used, including compounds of the invention. It is contemplated that linkers and methods described in our U.S. Pat. No. 6,573,337 which is the U.S. national stage application of International Patent Application No. PCT/AU97/00544, are also suitable for use with the compounds of this invention.
Thus in a fourth aspect the invention provides a linker-saccharide complex, comprising a linker group and a starting compound comprising a protecting group of general formula I or II as defined above. Any suitable linker compound may be used, including compounds of the invention. Again, it is contemplated that linkers and methods described in our U.S. Pat. No. 6,537,332, which is the U.S. national stage application of International Patent Application No. PCT/AU97/00544, may be used.
In a fifth aspect the invention provides a method of solution phase synthesis of oligosaccharides, comprising the step of sequentially linking mono- or oligosaccharide groups to a linker-saccharide complex as described above.
These methods are particularly useful for combinatorial synthetic applications. The solution phase method of the invention may, for example, be used for combinatorial synthesis of aminoglycoside compounds.
The invention also provides kits useful in solution phase synthesis or combinatorial synthesis of oligosaccharides or peptides, comprising either
a) a resin-linker-saccharide or resin-linker-peptide (or amino acid) support,
b) a linker-saccharide or linker-peptide (or amino acid) complex, or
c) a resin-linker support, according to the invention, as described above.
For peptide synthesis it may be convenient in some circumstances to start with a resin-linker-amino acid support or linker-amino acid complex, while in others a starter peptide may more suitably be provided in the support or linker complex. The kit may optionally also comprise one or more further reagents such as protecting agents, deprotecting agents, and/or solvents suitable for solid phase or combinatorial synthesis. The person skilled in the art will be aware of suitable further reagents. Different types of kit can then be chosen according to the desired use.
The invention also provides a kit useful in solid phase synthesis or combinatorial synthesis of oligosaccharides, comprising a linker-saccharide complex according to the invention, as described above. The kit may optionally also comprise one or more further reagents such as protecting agents, deprotecting agents, and/or solvents suitable for solid phase or combinatorial synthesis. The person skilled in the art will be aware of suitable further reagents. Different types of kit can then be chosen according to the desired use.
For the purposes of this specification it will be clearly understood that the word xe2x80x9ccomprisingxe2x80x9d means xe2x80x9cincluding but not limited toxe2x80x9d, and that the word xe2x80x9ccomprisesxe2x80x9d has a corresponding meaning.
Abbreviations used herein are as follows:
The invention will now be described in detail by way of reference only to the following non-limiting examples, in which the structures of individual compounds are as summarised in the following tables and structures.
We have now developed a novel enamine-type protective system, including the preparation of reagents, and methods for selective amino group protection and deprotection. This has been illustrated by synthesizing a number of 5-acyl-1,3-dimethyl-barbituric acids (ADA) (Examples 1-11). During the syntheses only C-acyl products were formed; no O-acylation was observed. The 5-acylation of 1,3-dimethylbarbituric acid was successfully carried out using carboxylic acids in the presence of DCC and DMAP (Examples 5 to 9). The more reactive acyl chlorides (Examples 3 to 4) and anhydrides (Examples 1 to 2) were also used, giving the same products in a DMAP-catalyzed reaction. Trichloroacetonitrile was used to construct a similar structure in the present of DBU (Example 10).
The 5-acyl-1,3-dimethylbarbituric acids were easily crystallized from polar solvents, avoiding the need for chromatographic purifications. These reagents are very cheap and easy to synthesize in a single reaction from the readily available 1,3-dimethyl-barbituric acid. We have used the 5-acyl-1,3-dimethylbarbituric acid reagents to prepare a wide variety of protected primary alkylamines (Examples 12-13), aminosugars (Examples 22 to 28) and amino acids (Examples 14 to 16).
The ADA-protected aminosugars can be used as aminosugar acceptors and aminosugar donors for solid or solution phase oligosaccharide synthesis. The ADA-protected amino acids are particularly useful as reagents for solid-phase peptide and glycopeptide syntheses, because they are unable to form oxazolones during the coupling reactions. Thus, no racemization can occur during the peptide bond formation (racemization can only occur in base-catalyzed proton abstraction). The ADA-protection is ideally orthogonal to the Boc-protection and quasi-orthogonal to the Fmoc system.
We have demonstrated that the system can be used for the protection of hydroxylamines (Example 17), hydrazines (Example 19) and hydrazides (Example 18). The vinylogous amide protection of amino groups was efficiently achieved by simply refluxing the unprotected amines with the precursor (5-acyl-1,3-dimethylbarbituric acid) in abs EtOH.
The ADA-protected derivatives are very stable in a wide range of reactions and work-up conditions. Different reagents (NH3, N2H4, NH2OH, nxe2x80x94BuNH2, BnNH2, NHxe2x80x94NHCOCH3, N2H4xAcOH, NaOH) have been developed for the cleavage of the protecting groups (Examples 17 to 20). The speed of protection and cleavage depends on the electronic and steric effects of the 5-acyl functional group.
We have also synthesized bifunctional 5-acyl-1,3-dimethylbarbituric acids (Example 11), which can be used as linkers for solid phase organic chemistry. We have successfully immobilized a bifunctional 5-acyl-1,3-dimethylbarbituric acid producing a xe2x80x9cresin-linker conjugatexe2x80x9d (Example 35). We have proved that this xe2x80x9cresin-linker conjugatexe2x80x9d was suitable for solid phase oligosaccharide synthesis by immobilizing a monosaccharide (Example 32), deprotecting its hydroxyl groups (Example 33) and later realising it during the cleavage (Example 33). We have demonstrated that the resin-linker conjugate was reusable, regenerating the original hydroxyl function with aqueous base treatment (Example 36). Alternatively the xe2x80x9camino-substituted resin-linker conjugatexe2x80x9d itself may be used for the next immobilization (Example 34).
The introduction of another reactive centre into the protecting group makes the system more flexible. Using 5-chloroacetyl-1,3-dimethylbarbituric acid, we have synthesised a chiral carbohydrate containing reagent (Example 31) for protection of organic compounds bearing an amino functionality. These types of molecules are especially suitable for resolution of enantiomers.
The 5-trichloroacetimino-1,3-dimethyl-barbituric acid gave rare 1,1-elimination in the reaction with primary amines, affording a novel type of compound (Example 29).